Scroll down this page and it shows you the offenders and how many kb you can save on them. Clicking on "Analyze JPEG" takes you to the optimized image - the work is actually done for you by WPT - choose the one with the smallest file size, save to your computer, rename to match what is on the server and upload there to overwrite.

Also, get rid of any png images you have that aren't transparencies - make them into compressed progressive jpegs to save still more kb and thus, get still more speed.

All this will speed your site up some more, lower your speed index number too, it will also get rid of that F grade for image compression.

It's effectively the average time when pixels were rendered. So a 469 means that on average the page content was displayed at 469ms (it doesn't mean that at 469ms specifically anything happened, it's just the average).

It's an absolute time and not referenced against anything else (just like load time or start render).